In copending application Ser. No. 912,268, filed June 5, 1978 by Daniel M. Cap and William H. Lake, titled HIGH PRESSURE METAL VAPOR DISCHARGE LAMPS OF IMPROVED EFFICACY, which is a continuation-in-part of an earlier application Ser. No. 812,479, filed July 5, 1977, similarly assigned, useful and efficient high pressure discharge lamps are disclosed having much smaller sizes than have been considered practical heretofore, namely discharge volumes of one cubic centimeter or less. In preferred form achieving maximum efficacy, these high intensity lamps utilize generally spheroidal thin-walled arc chambers together with vapor pressures above 5 atmospheres and reaching progressively higher levels as the size is reduced. The convective arc instability usually associated with the high pressures utilized is avoided and there is no appreciable hazard from possibility of explosion. Practical designs provide wattage ratings or lamp sizes starting at about 100 watts and going down to less than 10 watts, the lamps having characteristics including color rendition, efficacy, maintenance and life duration making them suitable for general lighting purposes.
High pressure metal vapor lamps have certain inherent shortcomings which persist even in miniature sizes. One of these is the delay in achieving full brightness after ignition, caused by the need to heat up the envelope and vaporize the metallic fill. This delay may be termed the cold start delay. Another is the even longer delay, termed the hot restart delay, which occurs should there be a momentary interruption of power to the lamp. The interruption may be due to a line outage, as frequently caused by lightning, or to a person switching off the lights and then changing his mind. The lamp then becomes extinguished and relighting will not occur immediately upon restoration of power. It is necessary first for the lamp to cool down and the metal vapor pressure to diminish before the ballast can restrike the arc, and then more time is required for the arc tube to heat up to full brilliance.
It is known to use a separate standby incandescent lamp in combination with a discharge lamp and a control circuit to supplement the light from the discharge lamp during its off or low illumination periods and thereby achieve instant light. Such a system is disclosed in Swiss Pat. No. 377,937 (Leuenberger, 1964) in which the standby lamp is energized by a relay whose winding receives two oppositely directed voltages derived from the circuit of a mercury vapor lamp. During both the cold start and the hot restart intervals, the vector difference of the two voltages is large enough to energize the relay and switch on the standby lamp, while during normal operation, the vector difference is too small and the standby lamp remains switched off. Another example is described in Swiss Pat. 444,305 (Vogeli, 1967) wherein the relay is replaced by a silicon controlled rectifier connected in series with the standby lamp across the power supply. Yet other examples are disclosed in U.S. Pat. No. 3,517,254 (McNamara, Jr., 1970) which uses a diac connected in series with the standby lamp to control the current flow through it, and U.S. Pat. No. 3,737,720 (Willis, 1973) which uses a pair of relays for the control function. In all of these prior art systems, the control circuitry is relatively elaborate and expensive.
It is also known to mount an arc tube and a filament in a sealed vitreous envelope wherein the filament serves as a ballast for the arc tube. A well-known example is the common household type sun lamp which includes a ballast filament and a filamentary electrode within the arc tube connected in series with the arc gap, along with a thermal switch which starts the lamp by short-circuiting the filamentary electrode. Another example is described in U.S. Pat. No. 2,899,583 (Macksoud, 1959) and comprises an arc tube and a filament including two portions all connected in series within a sealed vitreous envelope. The arc tube starts with both filament portions connected in series, but during normal operation heat from the arc tube causes a thermal switch to close and short circuit one of the portions. Such prior art lamps using a filament within the sealed outer envelope for a ballast are relatively low efficacy lamps. Although the ballasting filament in these lamps does give some light, it is generally operated at a low temperature in order to lengthen its life and make it more commensurate with that of the arc tube which may exceed 5,000 hours. As a result, the filament does not give much light and does not truly perform the role of a standby light source assuring substantially immediate light when the lamp is switched on, or a quick relighting in the event of an interruption.